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          <span data-v-64685f0e>2020 - </span>
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          <span data-v-64685f0e>2020 - </span>
          2021
        </a></span></div></div> <div data-v-2d5f533b><main class="page"><div class="page-title" style="display:none;"><h1 class="title">第十二章 BOM</h1> <div data-v-3b7f5bdf><i class="iconfont reco-account" data-v-3b7f5bdf><span data-v-3b7f5bdf>伊水河</span></i> <i class="iconfont reco-date" data-v-3b7f5bdf><span data-v-3b7f5bdf>2020-12-25</span></i> <i class="iconfont reco-eye" data-v-3b7f5bdf><span id="/note/professionalJavascript4/12.html" data-flag-title="Your Article Title" class="leancloud-visitors" data-v-3b7f5bdf><a class="leancloud-visitors-count" style="font-size:.9rem;font-weight:normal;color:#999;"></a></span></i> <i class="iconfont reco-tag tags" data-v-3b7f5bdf><span class="tag-item" data-v-3b7f5bdf>javascript高级程序设计</span></i></div></div> <div class="theme-reco-content content__default" style="display:none;"><h2 id="window"><a href="#window" class="header-anchor">#</a> window</h2> <h3 id="窗口关系"><a href="#窗口关系" class="header-anchor">#</a> 窗口关系</h3> <p><code>window</code>属性：</p> <ul><li>top：指向最上（外）层窗口，即浏览器窗口本身。</li> <li>parent：指向当前窗口的父窗口。若当前窗口是最上层窗口，则<code>parent</code>等于<code>top</code>等于<code>window</code>。</li> <li>self：始终指向<code>window</code>。实际上<code>self</code>和<code>window</code>是同一个对象，其出现目的是为了和<code>top</code>、<code>parent</code>保持一致</li> <li>name：窗口名称，若最上层窗口不是由<code>window.open()</code>打开的，那么其<code>name</code>不会包含值。</li></ul> <h3 id="像素比"><a href="#像素比" class="header-anchor">#</a> 像素比</h3> <p>CSS像素是Web开发中使用的统一像素单位。这个单位的背后其实是一个角度：0.0213°。如果屏幕距离人眼是一臂长，则以这个角度计算的CSS像素约为1/96英寸（0.0264cm）。</p> <p>这样定义像素大小是为了在不同设备上统一标准。比如，低分辨率平板设备上12像素的文字应该与高清4K屏幕下12像素的文字具有相同的大小。这样就会有个问题，不同像素密度的屏幕下就会有不同的缩放系数，以便把物理像素（屏幕实际分辨率）转换为CSS像素（浏览器报告的虚拟分辨率）。举个例子，手机屏幕的物理分辨率为1920×1080，但因为其像素可能非常小，所以浏览器就需要将其分辨率降为较低的逻辑分辨率，比如640×320。这个物理像素与CSS像素的转换比率由<code>window.devicePixelRatio</code>属性提供。上栗其值为3。这样一来，12像素的文字实际上是用36像素的物理像素来显示的。</p> <p><code>window.devicePixelRatio</code>实际上与每英寸像素数（DPI）是对应的。DPI表示单位像素密度。</p> <h3 id="打开新窗口"><a href="#打开新窗口" class="header-anchor">#</a> 打开新窗口</h3> <p><code>window.open()</code>可以打开一个新的窗口，返回对新窗口的引用：</p> <div class="language-javascript extra-class"><pre class="language-javascript"><code><span class="token keyword">let</span> win<span class="token operator">=</span>window<span class="token punctuation">.</span><span class="token function">open</span><span class="token punctuation">(</span><span class="token string">&quot;http://win.com&quot;</span><span class="token punctuation">,</span><span class="token string">&quot;win&quot;</span><span class="token punctuation">,</span><span class="token string">&quot;height=400,width=400&quot;</span><span class="token punctuation">)</span>
</code></pre></div><p>关闭窗口，在返回的引用上使用close()</p> <div class="language-javascript extra-class"><pre class="language-javascript"><code>win<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
</code></pre></div><p>这个方法只能用于window.open()创建的窗口，弹出的窗口可以调用top.close()来关闭自己。关闭窗口后，引用依然存在，但是只能用于检查其closed属性了：</p> <div class="language-javascript extra-class"><pre class="language-javascript"><code><span class="token function">alert</span><span class="token punctuation">(</span>win<span class="token punctuation">.</span>closed<span class="token punctuation">)</span> <span class="token comment">// true</span>
</code></pre></div><p>新创建（弹出）的窗口window上有一个opener属性，指向创建它的窗口。这个属性只在最上层window（top）对象上存在。</p> <div class="language-javascript extra-class"><pre class="language-javascript"><code><span class="token function">alert</span><span class="token punctuation">(</span>win<span class="token punctuation">.</span>opener<span class="token operator">===</span>window<span class="token punctuation">)</span> <span class="token comment">// true</span>
</code></pre></div><p>窗口不会记录自己打开的新窗口，需要开发者自己记录。把opener设置为null会切断它们之间的联系，且无法恢复。</p> <p>浏览器会在用户操作下才允许创建弹窗，通常在鼠标点击或按下键盘中某个键时才能打开。所有现代浏览器都内置了屏蔽弹窗的程序，因此大多意料之外的弹窗都会被屏蔽。若弹窗被屏蔽了，window.open()可能会返回null或抛出错误，所以可通过下面的方法来判断弹窗是否被屏蔽：</p> <div class="language-javascript extra-class"><pre class="language-javascript"><code><span class="token keyword">let</span> blocked<span class="token operator">=</span><span class="token boolean">false</span><span class="token punctuation">;</span>
<span class="token keyword">try</span><span class="token punctuation">{</span>
    <span class="token keyword">let</span> win<span class="token operator">=</span>window<span class="token punctuation">.</span><span class="token function">open</span><span class="token punctuation">(</span><span class="token string">&quot;http://win.com&quot;</span><span class="token punctuation">,</span><span class="token string">&quot;_blank&quot;</span><span class="token punctuation">)</span>
    <span class="token keyword">if</span><span class="token punctuation">(</span>win<span class="token operator">==</span><span class="token keyword">null</span><span class="token punctuation">)</span> blocked<span class="token operator">=</span><span class="token boolean">true</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token keyword">catch</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">{</span>
    blocked<span class="token operator">=</span><span class="token boolean">true</span>
<span class="token punctuation">}</span>
<span class="token keyword">if</span><span class="token punctuation">(</span>blocked<span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token comment">// 证明被屏蔽，做一下什么</span>
<span class="token punctuation">}</span>
</code></pre></div><h2 id="location"><a href="#location" class="header-anchor">#</a> location</h2> <p>location是window的属性，也是document的属性，它们指向同一个对象。</p> <p>用location来设置URL通常会使用：</p> <div class="language-javascript extra-class"><pre class="language-javascript"><code>location<span class="token operator">=</span><span class="token string">&quot;url&quot;</span>
location<span class="token punctuation">.</span>herf<span class="token operator">=</span><span class="token string">&quot;url&quot;</span>
<span class="token comment">// 它们与显式调用assign()是一样的。</span>
location<span class="token punctuation">.</span><span class="token function">assign</span><span class="token punctuation">(</span><span class="token string">&quot;url&quot;</span><span class="token punctuation">)</span>
</code></pre></div><p>除了hash之外，只要修改location的一个属性，都会导致页面重新加载新的URL。</p> <h2 id="history"><a href="#history" class="header-anchor">#</a> history</h2> <p>history对象表示当前窗口首次使用以来，用户的导航历史记录。每个window都有自己的history对象出于安全考虑，这个对象不会暴露用户访问过的URL，但可以通过它来前进、后退。</p> <h3 id="方法"><a href="#方法" class="header-anchor">#</a> 方法</h3> <ul><li>pushState(state, title[, url])：向当前浏览器会话的历史堆栈中添加一个状态（state）。请注意，浏览器不会在调用pushState() 之后尝试加载此URL，但可能会稍后尝试加载URL，例如在用户重新启动浏览器之后。 新的URL不必是绝对的。 如果是相对的，则相对于当前URL进行解析。 新网址必须与当前网址相同 origin； 否则，pushState()将引发异常。 如果未指定此参数，则将其设置为文档的当前URL。</li> <li>replaceState(stateObj, title[, url])：修改当前历史记录实体，如果你想更新当前的state对象或者当前历史实体的URL来响应用户的的动作的话这个方法将会非常有用。</li> <li>go([num])：正数前进，负数后退。不传值或传0，相当于<code>location.reload()</code>。</li> <li>back()：相当于go(-1)</li> <li>forward()：相当于go(1)</li></ul> <h3 id="事件"><a href="#事件" class="header-anchor">#</a> 事件</h3> <ul><li>hashchange：该事件会在URL后面的hash值变化时被触发。</li> <li>popstate：当活动历史记录条目更改时，将触发<code>popstate</code>事件。如果被激活的历史记录条目是通过对<code>history.pushState()</code>的调用创建的，或者受到对<code>history.replaceState()</code>的调用的影响，popstate事件的state属性包含历史条目的状态对象的副本。需要注意的是调用<code>history.pushState()</code>或<code>history.replaceState()</code>不会触发popstate事件。只有在做出浏览器动作时，才会触发该事件，如用户点击浏览器的回退按钮（或者在Javascript代码中调用<code>history.back()</code>或者<code>history.forward()</code>方法）</li></ul> <div class="language-javascript extra-class"><pre class="language-javascript"><code>window<span class="token punctuation">.</span><span class="token function">addEventListener</span><span class="token punctuation">(</span><span class="token string">'popstate'</span><span class="token punctuation">,</span><span class="token parameter">event</span><span class="token operator">=&gt;</span><span class="token punctuation">{</span>
    <span class="token comment">// 为pushState或replaceState传入的state值</span>
    <span class="token keyword">let</span> state<span class="token operator">=</span>event<span class="token punctuation">.</span>state<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre></div><blockquote><p>更多参考 <a href="https://developer.mozilla.org/zh-CN/docs/Web/API/History_API" target="_blank" rel="noopener noreferrer">History API<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></p></blockquote></div> <footer class="page-edit" style="display:none;"><!----> <!----></footer> <!----> <!----> <!----></main> <!----></div></div></div></div><div class="global-ui"><div class="back-to-ceiling" style="right:1rem;bottom:6rem;width:2.5rem;height:2.5rem;border-radius:.25rem;line-height:2.5rem;display:none;" data-v-c6073ba8 data-v-c6073ba8><svg t="1574745035067" viewBox="0 0 1024 1024" version="1.1" xmlns="http://www.w3.org/2000/svg" p-id="5404" class="icon" data-v-c6073ba8><path d="M526.60727968 10.90185116a27.675 27.675 0 0 0-29.21455937 0c-131.36607665 82.28402758-218.69155461 228.01873535-218.69155402 394.07834331a462.20625001 462.20625001 0 0 0 5.36959153 69.94390903c1.00431239 6.55289093-0.34802892 13.13561351-3.76865779 18.80351572-32.63518765 54.11355614-51.75690182 118.55860487-51.7569018 187.94566865a371.06718723 371.06718723 0 0 0 11.50484808 91.98906777c6.53300375 25.50556257 41.68394495 28.14064038 52.69160883 4.22606766 17.37162448-37.73630017 42.14135425-72.50938081 72.80769204-103.21549295 2.18761121 3.04276886 4.15646224 6.24463696 6.40373557 9.22774369a1871.4375 1871.4375 0 0 0 140.04691725 5.34970492 1866.36093723 1866.36093723 0 0 0 140.04691723-5.34970492c2.24727335-2.98310674 4.21612437-6.18497483 6.3937923-9.2178004 30.66633723 30.70611158 55.4360664 65.4791928 72.80769147 103.21549355 11.00766384 23.91457269 46.15860503 21.27949489 52.69160879-4.22606768a371.15156223 371.15156223 0 0 0 11.514792-91.99901164c0-69.36717486-19.13165746-133.82216804-51.75690182-187.92578088-3.42062944-5.66790279-4.76302748-12.26056868-3.76865837-18.80351632a462.20625001 462.20625001 0 0 0 5.36959269-69.943909c-0.00994388-166.08943902-87.32547796-311.81420293-218.6915546-394.09823051zM605.93803103 357.87693858a93.93749974 93.93749974 0 1 1-187.89594924 6.1e-7 93.93749974 93.93749974 0 0 1 187.89594924-6.1e-7z" p-id="5405" data-v-c6073ba8></path><path d="M429.50777625 765.63860547C429.50777625 803.39355007 466.44236686 1000.39046097 512.00932183 1000.39046097c45.56695499 0 82.4922232-197.00623328 82.5015456-234.7518555 0-37.75494459-36.9345906-68.35043303-82.4922232-68.34111062-45.57627738-0.00932239-82.52019037 30.59548842-82.51086798 68.34111062z" p-id="5406" data-v-c6073ba8></path></svg></div><APlayer audio="" fixed="true" theme="#b7daff" loop="loop" order="random" preload="auto" volume="0.7" mutex="true" lrc-type="3" list-folded="true" list-max-height="250" storage-name="vuepress-plugin-meting" id="aplayer-fixed"></APlayer></div></div>
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